YBCO crystals

Figure 4.23. A typical PIXE spectrum from the scanned area on the base YBCO crystal. (Reproduced by permission of Faiz et al. 1996.)...

A certain similarity to the Norton method can be recognized in the combinec technique for growing YBCO crystals from a high-temperature melt [386], where electrolysis is used for locally increasing the oxygen concentration and thus foi promoting nucleation in melts saturated with respect to CuO. 

Resistivity as a Function of Temperature and Oxygen Content. The in-plane (pa) as well as the out-of-plane (pc) resistivity of detwinned YBCO crystals... 

Twin planes are usually introducted into YBCO crystals at the transformation from tetragonal to orthorhombic S3rmmetry. The twin planes only act as effective pinning centers, when the fluxoids are aligned parallel to the twin plane while the Lorentz force acts perpendicular to the plane. Twin planes are not dominant pinning centers in melt-processed YBCO samples. 

Faiz et al. (1996) have applied micro-PIXE analysis to study solute distributions in a single crystal sample of YiBa2Cu307 5 high temperature superconductor (YBCO) of dimensions 1.3 mm x 1.5 mm x 75 pm. It contained a small secondary crystal overgrowth of dimensions 340 x 340 x 100 pm3. The interface region between the smaller crystal and the base crystal was covered with a material which appeared to be residual flux. The instrument employed a 2.5 MeV focused proton beam of about 4 pm resolution, which could scan an area of 500 x 500 pm2 on the sample surface. The microbeam current was kept low (typically about 30 pA) to avoid any damage to the sample. 

In fig.l there are presented results of detailed analysis of resistive behavior for YBCO single crystal in the model where total resistivity ptot is... 

Figure V 3 l. Temperature dependence of the in-plane resistivity for YBCO single crystal [4]...

According to the experimental results obtained on untwinned YBCO single crystals [1, 2, 3], temperature dependence of resistivity in the planes is linear, i.e. ppiane = d + 0-2T, where a and <22 are constants. On the other hand, temperature dependence of the chain resistivity in YBCO was found to follow approximately a quadratic dependence, i.e. Pchain = b +, where bi and 62 are constants [3]. T2 dependence... 

Figure 2.20. Comparison of the crystal structures of Ocft-right) LSCO, YBCO, and BSCCO superconductors, respectively. Reproduced with permission from Prof. Hoffman s webpage at Harvard University http //hoffman.physics.harvard.edu/research/SCmaterials.php...

It was found recently that for a lithium-doped YBCO the dependence of on the lithium content has a maximum corresponding to one Li atom per molecule [311]. Unusual properties of the lithium-doped LCO are described [278]. It is evident that a wide range of these materials can be obtained by electrochemical methods. Diffusion coefficients Dli in solid cuprate phases demonstrate the same tendencies as for single crystals, Du is lower than for powders [308], strongly depending on stoichiometry [304,305], The highest room-temperature Du values described are close to lO" cm /s. ... 

The problem of lateral modification of HTSC surface layers, and the local electrosynthesis of HTSCs on the surface of patterned substrates including the precursors is very interesting. Such processes can occur, for example, during electrooxidation of metals when the process in its initial stages takes place only on isolated microscopic regions. Thus, Josephon junctions on the surface of Bi-Sn alloys [222] and on ceramic YBCO samples [295,444] were obtained by using electrochemical oxidation without any special local techniques. But it is hard to control such oxidation processes, and sufficient reproducibility cannot be ensured for most systems. Josephson tunnel junctions based on electrochemically synthesized BKBO crystals have been described [445]. 

Only a few studies [453,543-547] have been carried but on the photoelectrochemical behaviour of YBCO ceramics and single crystals. In nonaqueous media, in the absence of degradation, the data show that the usual concepts of photoelectrochemistry of p-type semiconductors apply to those systems. Such studies have not yet been actively developed, probably because of the high sensitivity of photoelectrochemical processes to the nature of materials. Reproducibility of data for such complex systems as HTSC oxides is an issue. At the same time, the photoassisted electrochemical processes on HTSC electrodes can lay the basis for certain effective technologies. This is especially true for the photoelectrochemical etching and metallization which prove to be extremely effective as applied to semiconductors [503,504]. 

Fig. 10.1. Critical current densities Jc for grain boundaries in YBCO films grown by epitaxy on suitably treated single crystal or bicrystal substrates, plotted against the [001] misorientation angle [10.4, 10.7, 10.51, 10.59, 10.60].

Another possibility for obtaining CSLs exists by approximating the crystal structure by pseudocubic or tetragonal unit cells. This approach of applying the CSL to non-cubic systems has been discussed in the literature by means of the constrained coincident site lattice [10.12] which has, among others, also been applied to YBCO grain boundaries. 

Fig. 10.7. HREM image of grain boundary in polycrystalline YBCO. The grain boundary appears free of impurity phases and is formed on the basal plane of one crystal, which borders a high-index plane in the second grain.

Fig. 14.2. Critical current density as a function of misorientation angle of [001]-tilt YBCO boundaries on SrTiOs bi-crystal substrates. Data are extracted from [14.1-14.4].

Fig. 14.3. Schematic illustration of YBCO grains which have nucleated on a surface where the interfacial interactions can be neglected. The (100), (010) and (001) low-energy crystallographic planes of YBCO constitute the boundary surfaces of the crystals.

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